1. Field of the Invention
The invention relates to a method and precursors for forming low dielectric constant insulator material in integrated circuits.
2. Statement of the Problem
As semiconductor technology advances, circuit elements and interconnections on wafers or silicon substrates become increasingly more dense. As a result of the continuing trend toward higher device densities, parasitic interdevice currents are increasingly problematic. In order to prevent unwanted interactions between these circuit elements, insulator-filled gaps, or trenches, located between active circuit devices and metallized interconnect layers are provided to physically and electrically isolate the elements and conductive lines.
For example, in metal-oxide-semiconductor (xe2x80x9cMOSxe2x80x9d) technology, it is necessary to provide an isolation structure that prevents parasitic channel formation between adjacent devices, such devices being primarily NMOS and PMOS transistors or CMOS circuits. Trench isolation technology has been developed in part to satisfy such insulation needs. The trenches are subsequently refilled with a dielectric, such as silicon dioxide, typically by a chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) technique.
Similar isolation techniques are used to separate closely spaced circuit elements that have been formed on or above a semiconductor substrate during integrated circuit fabrication. The circuit elements may be active devices or conductors, and are isolated from each other by refilled xe2x80x9cgapsxe2x80x9d.
The capacitance across a gap is governed by the formula
C=∈rkA/t,
where C is the capacitance, ∈r is the relative dielectric constant of the gap fill material, k is a constant, A is the area of the gap (i.e., the area of the side of the circuit element forming the gap), and t is the thickness or width of the gap. As gap widths decrease with increasing density, the capacitance across the dielectric gap fill material increases. Thus, as integrated circuits become increasingly dense, decreasing t, it is necessary to lower the dielectric constant of the gap fill material to reduce cross-talk, capacitive coupling and resulting speed degradation, and power consumption. To compensate for smaller gap dimensions, it is known to substitute dielectric materials having dielectric constants lower than silicon dioxide.
In damascene process techniques, the dielectric material is deposited as a blanket, either without or with minimal gap-fill requirements. Trenches or vias are then etched in this material, and these are filled with conductor material. Although the dielectric material is deposited first, and conductive material is used to fill the gaps, the need for a low dielectric blanket material between circuit elements is similar to the described requirement for a low-dielectric constant gap-fill material.
In addition to filling gaps, or trenches, between active devices or local interconnects, there is also a need for a low-dielectric constant insulator material for use as interlayer dielectric (xe2x80x9cILDxe2x80x9d) and other insulating layers in integrated circuit devices. Thus, there is a need for a method of depositing insulating material having a relative dielectric constant, ∈r of 3.0 or less.
It is known that carbon-containing silicon oxide films have lower dielectric constants than silicon oxide films. It is believed that the carbon works by decreasing the effective density of the film, since a film of zero density, that is, a vacuum, has a dielectric constant of 1. Also, carbon contained in silicon oxide films, for example, in CH3 groups, is usually less polarizable than silicon oxide, thereby lowering the capacitance, or dielectric constant, of the thin film. It is, therefore, known in the art to deposit a carbon-containing silicon oxide film by reacting an organic carbon-containing gaseous precursor compound and a silicon-containing gaseous precursor compound in a PECVD or HDP-CVD process. Typically, a single type of precursor compound contains both silicon and an organic carbon-containing group. For example, it is known to react a gaseous organo silane, such as a methyl silane, in the presence of nitrous oxide in a PECVD process to form a carbon-containing silicon oxide dielectric film. Whereas PECVD SiO2 typically has a measured ∈r of 4.0-4.2, SiO2 doped with carbon can have a reduced her of less than 3.0. The economic feasibility of a PECVD process is determined by, among other factors, the deposition rate. Carbon-containing a SiO2 films have deposition rates that increase as a function of increasing high-frequency radio frequency (xe2x80x9chf rfxe2x80x9d) power, and as a function of increasing nitrous oxide, N2O, flow. Increasing hf rf power, however, has a detrimental effect on wafer uniformity, and increasing nitrous oxide flow reduces the amount of carbon in the film, thereby increasing the dielectric constant, ∈r.
Design feature widths of integrated circuit devices are currently approaching 0.25 xcexcm, or 250 nm. To achieve corresponding overall circuit density, gap dimensions of approximately 100 nm to 400 nm gap width range and 300 nm to 1000 nm gap depth range are desired, having a corresponding range of aspect ratios of 2 to 6. A gap opening of 500 nm or less is too small for depositing material using conventional CVD and PECVD methods. Currently, high density plasma (xe2x80x9cHDPxe2x80x9d) CVD is used to fill high aspect ratio gaps. Also, using HDP-CVD, it is usually possible to deposit silicon oxide films at lower temperatures (e.g., 150xc2x0 C. to 250xc2x0 C.) than in a PECVD process. It is known that the addition of carbon to a SiO2 dielectric film formed by an HDP-CVD process also lowers the dielectric constant of the dielectric material. HDP-CVD processes typically use a gas mixture containing oxygen, silane, and inert gases, such as argon. In an HDP process, an rf bias is applied to a wafer substrate in a reaction chamber. Some of the gas molecules, particularly argon, are ionized in the plasma and accelerate toward the wafer surface when the rf bias is applied to the substrate. Material is thereby sputtered when the ions strike the surface, resulting in simultaneous deposition and etching of the dielectric film. This simultaneous deposition and etching is a problem because silicon-carbon bonds, for example, Sixe2x80x94CH3 bonds, are less robust than Sixe2x80x94O bonds of silicon oxide. The undesired result is that carbon is preferentially etched from the deposited dielectric film, causing the dielectric constant of the final film to be higher.
In a method in accordance with the invention, the reactant gas mixture in a plasma-enhanced CVD process includes a hydrogen-containing chemical species for forming Sixe2x80x94H bonds in a novel carbon-containing silicon oxide film in which both Sixe2x80x94C and Sixe2x80x94H bonds are present. Typically, the CVD reactant gas mixture contains silicon, oxygen, hydrogen and carbon atoms. Because dielectric material deposited in accordance with the invention has a significant number of Sixe2x80x94H bonds, which are more robust than Sixe2x80x94C bonds, it is more resistant to undesired etching, oxidation and other physical changes during fabrication than dielectric material formed by conventional methods. A dielectric film formed in accordance with the invention typically has a dielectric constant less than 3. The invention provides a novel method for rapidly depositing in an integrated circuit a novel, low dielectric constant film having enhanced uniformity characteristics. The dielectric film in accordance with the invention may be viewed as a silicon oxide film having Sixe2x80x94C and Sixe2x80x94H bonds, as well as Sixe2x80x94O bonds.
The invention may be used to deposit a premetal dielectric layer (xe2x80x9cPMDxe2x80x9d), an intermetal dielectric layer (xe2x80x9cIMDxe2x80x9d), an interlayer dielectric layer (xe2x80x9cILDxe2x80x9d), a passivation layer and other insulator films requiring a low dielectric constant in an integrated circuit. The invention is most useful when applied to deposit a thin film of low dielectric constant material in a gap, or trench, between active devices or conductive interconnects in high density integrated circuits. It is also useful, however, in damascene process techniques.
A method for forming a dielectric film in accordance with the invention comprises steps of: providing a reactant gas mixture in a reaction chamber, the reactant gas mixture containing silicon, oxygen, carbon, and hydrogen atoms; and reacting the reactant gas mixture in the reaction chamber to deposit dielectric material on the substrate. In a first general embodiment, the reactant gas mixture comprises a carbon-containing precursor compound and a hydrogen-containing precursor compound. Usually, the carbon-containing precursor compound and the hydrogen-containing precursor compound are not identical. Preferably, the hydrogen-containing precursor compound contains no carbon atom. Typically, the hydrogen-containing compound is silane, having a stoichiometric formula SiH4. Typically, the carbon-containing precursor compound is an alkyl silane, such as trimethyl silane and tetramethyl silane. Preferably, the reactant gas mixture comprises SiH4 and an alkyl silane.
In a second general embodiment, the reactant gas mixture comprises a carbon-containing precursor compound and a hydrogenating gas. Typically, the hydrogenating gas is hydrogen-gas, H2. Typically, the carbon-containing precursor compound contains an alkyl group. In a particular variation of the embodiment, the carbon-containing precursor compound comprises a silicon atom, an oxygen atom, and a carbon atom, the oxygen atom being chemically bonded to the silicon atom. For example, the carbon-containing precursor compound may be octamethylcyclotetrasiloxane, corresponding to the stoichiometric formula (SiO(CH3)2)4.
Preferably, a method in accordance with the invention includes using a plasma-enhanced CVD technique to form a dielectric film on a substrate. A plasma-enhanced CVD technique in accordance with the invention may be plasma-enhanced CVD (xe2x80x9cPECVDxe2x80x9d) or high-density plasma CVD (xe2x80x9cHDP-CVDxe2x80x9d). Therefore, the reactant gas mixture in the CVD reaction chamber preferably comprises an inert plasma forming gas, such as argon. The reactant gas mixture may also comprise an inert carrier gas. The reactant gas mixture may also typically comprise organic solvent molecules. In PECVD, a high-frequency (xe2x80x9cHFxe2x80x9d) radio frequency (xe2x80x9crfxe2x80x9d) is applied to the reactant gas mixture to form the plasma. In HDP-CVD, a low-frequency (xe2x80x9cLFxe2x80x9d) rf is applied to the reactant gas mixture, and a HF rf is applied to the substrate wafer. PECVD may be used to deposit carbon-containing silicon oxide films in many applications, but usually not in so-called shallow trench isolation (xe2x80x9cSTIxe2x80x9d) or other aggressive gap fill applications, in which closely spaced, narrow gaps (trenches) are filled with low dielectric constant material. A method employing HDP-CVD is especially useful to fill isolator gaps having a gap width in the range of from 50 nm to 1000 nm and an aspect ratio in the range of from 1 to 3 or greater.